Shift elements for automatic transmissions are clutches or brakes and generally include a hydraulically or pneumatically actuable piston, which has a piston chamber in which the hydraulic medium or the pressure medium is situated, an axially movable clutch half which is operatively connected to the piston, and an axially static clutch half. Shift elements may also be actuated electromechanically.
From the prior art, it is known in the case of automatic transmissions for positively locking shift elements, in particular dog-clutch shift elements, to be used instead of frictional shift elements as shift elements. Advantageously, this results in with lower production costs and less structural space being required. Furthermore, the drag torques are reduced, whereby the transmission efficiency is increased.
Dog-clutch shift elements generally include an axially movable dog, which is operatively connected to the piston and which has a dog toothing, and a counterpart dog as clutch body with a dog toothing with which the dog toothing of the dog meshes when in the engaged state. The dog may in this case preferably be a sliding sleeve.
In the case of dog-clutch shift elements, a device for detecting the travel of the axially movable dog is generally provided, which detects the exact axial position of the axially movable dog after the triggering of the engaging of the dog. Based on the detection of the exact axial position of the axially movable dog, deviations from the set-point profile are correctable. A Hall sensor arranged in the bearing carrier of the dog-clutch shift element is preferably used for detecting the travel of the axially movable dog, wherein the dog has an encoder contour which is suitable for the Hall sensor and which is preferably configured in the manner of a notch. This travel detection can also be used analogously for frictionally locking shift elements.
In the case of Hall sensors, the signal is disadvantageously manipulated or falsified by metallic particles. Since “metallic dirt” forms in a transmission as a result of abrasion or as a result of the assembly process, the metallic dirt can influence the sensor signal. If a metallic particle is situated in front of the sensor, this manipulates the sensor signal such that the sensor indicates an axial position other than the true axial position of the axially movable dog.